1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image based on an image signal.
2. Description of the Related Art
Recently, there is a need to output a high-quality image from an image forming apparatus such as a printer or copying machine that have adopted the electrophotographic method. However, the image forming apparatus suffers uneven density called banding that occurs in the paper conveyance direction (sub-scanning direction) due to various factors in the printing mechanism. This uneven density largely affects the image quality.
The factors that cause uneven density include the mechanical factors of members concerning image formation. For example, the uneven rotation speed of a photosensitive member leads to the uneven density. The uneven rotation speed results from the uneven rotation of an electric motor that drives the photosensitive member or the decentering of the driving gear that transfers the driving force. If slow rotation and quick rotation of the photosensitive member are periodically repeated due to the uneven rotation speed of the photosensitive member, the position of an electrostatic latent image shifts at the time of exposure, or the transfer position shifts at the time of primary transfer from the photosensitive member to the intermediate transfer material. For this reason, a region where the image is densely formed on the intermediate transfer material and a region where the image is sparsely formed are repetitively generated. When this image is macroscopically observed, the region where the image is densely formed appears as high density. Conversely, the region where the image is sparsely formed appears as low density. As a result, a user recognizes it as periodical uneven density.
To solve this problem, Japanese Patent Laid-Open No. 2004-317538 proposes a technique of reducing uneven density by changing the exposure amount in accordance with image data so as to correct a position shift caused by the uneven rotation speed of a photosensitive member. Japanese Patent Laid-Open No. 2007-108246 proposes a technique of reducing uneven density by storing uneven density information, correcting the image density to cancel the uneven density, and then performing image forming processing.
However, in the above-described method of correcting the position shift or method of correcting the image density, if the maximum density of a pixel after correction exceeds 100%, the correction value is not reflected so the uneven density correction is not sufficient. This problem will be described here with reference to FIG. 20.
FIG. 20 illustrates a state in which image position correction processing is performed for dot 1, dot 2, and dot 3 located at positions i to (i+2) adjacent in the sub-scanning direction. The initial density value of the dots is 100%, as indicated by 2400. To suppress uneven density, the position of dot 2 is corrected by 0.01 dot upward in FIG. 20, and the position of dot 3 is corrected by 0.03 dot upward without correcting the position of dot 1, as indicated by 2401 to 2403.
Reference numerals 2404 to 2406 represent density distribution to each pixel when correcting the position. To correct the position of dot 2 by 0.01 dot upward in FIG. 20, correction is performed by shifting the center of gravity of dot 2 by 0.01 dot across two lines such that the density at the position i is 1%, and that at the position (i+1) is 99%, as indicated by 2405. Similarly, to correct the position of dot 3 by 0.03 dot upward in FIG. 20, correction is performed such that the density at the position (i+1) is 3%, and that at the position (i+2) is 97%, as indicated by 2406.
The final density after the correction is the sum of these densities. As indicated by 2407, the densities at the positions i to (i+2) are 101%, 102%, and 97%. However, since a dot whose density is more than 100% cannot be formed, the excess over 100% is truncated, and the actual densities at the positions i to (i+2) are 100%, 100%, and 97%. If the density after the correction exceeds 100%, the dot cannot be corrected to the desired position so the uneven density correction is insufficient. Image position correction has been described above. The same problem arises in the method of correcting the image density as well.